This invention relates to an embossing device with which a lateral edge of a thermo-plastic sheet material (hereinafter referred to as a film, when applicable) is embossed during the manufacture of the film to facilitate the handling of the film. In manufacturing films, a lateral edge of the film is embossed, or is physically deformed to improve the roll winding characteristic when the film is wound. It also facilitates the handling of the film when it is thereafter subjected to various processes.
An ordinary device for carrying out the physical deformation is generally called "an embossing device". In such a device, a film is inserted between two small rollers about 10 mm in width with minute teeth. In this technique, the film is clamped by the rollers under pressure and is roughened. In this case, the lateral edge of the film is deformed by the force applied to roughen the film, and as a result the lateral edge of the film oten forms "wavy pleats". The wavy pleats are of course poor in appearance, and at worst may greatly degrade the film's handling characteristic in winding or in the subsequent processing of the film.
One of the effects of the above-described embossing process is that if the embossing process is carried out while the film is being heated, then the variation with time of the roughness of the film surface is made lower. Therefore, in order to prevent such variations in the roughness of the film surface, it is desirable to subject the film to the embossing process while the film is being heated. However, this method is disadvantageous in that significant wavy pleats are created in the film. Various prior art techniques have been attempted to remove these pleats.
For example, U.S. Pat. No. 3,502,765 to Spencer (Japanese Patent Publication No. 47-16064) relates to a method of eliminating the above-mentioned wavy pleats and discloses a device wherein a film is partially heated just before the embossing process and is then thereafter shrunken. This compensates for the expansion of the film generated in the embossing process. A technique for partial heating is, however, required for this device and it is difficult to heat particular parts of the film. Furthermore, since it is mandatory to compensate for the expansion of the film generated in the embossing process, the amount of shrinkage must be controlled according to the variation of the force during the embossing process. The shrinkage due to heat is different for different kinds of films. Therefore, the heating temperature must be varied depending on the type of film. Various problems are raised in practical use with this technique.